Acceleration of heavy particles



March 19, 1968 El D. COURANT 3,374,378

' ACCELERATION OF HEAVY PARTCLES Original Filed Nov. 18, 1963 4| mdmmmmmmmmmmmmmmmmmmmmm F/'g.3a

INVENTOR.

Y ERNEST D. COURANT M LAJLMW,

United States Patent Otiice 3,374,378 ACCELERATION GF HEAVY PARTICLES Ernest D. Courant, Bayport, N.Y., assignor to the United States of America as represented by the United States Atomic Energy Commission Continuation of application Ser. No. 324,593, Nov. 18, 1963. This application July 14, 1967, Ser. No. 654,039 6 Claims. (Cl. 313-63) ABSTRACT F THE DISCLOSURE A method of increasing the revolution frequency and energy of heavy electrically charged particles that are magnetically held on a revolution orbit in a strong focusing synchrotron, wherein the particles are injected at a pre-selected low velocity from a linear accelerator and exposed to a high frequency electric eld the frequency of which is successively made equal to different harmonics of the revolution frequency in the synchrotron with decreasing ordinal numbers of the harmonics whereby the revolution frequency and energy of the particles can be increased in the multiple bev. range in the synchrotron with a narrow band of electric field frequencies.

Cross-reference to related applications This application is a continuation of application Ser. No. 324,593, tiled Nov. 18, 19613, and now abandoned, by Ernest D. Courant, and assigned to the assignee of this application.

Background of the invention This invention was made in the course of, or under a contract with the United States Atomic Energy Commission.

In high energy physics, it is desirable to accelerate charged particles that are heavier than protons. High energy deuteron beams are useful, for example, in producing neutrons since a deuteron is a rather loosely bound combination of a neutron and a proton and is likely to break up in a stripping action when it hits the target. In this case a considerable frac-tion of the collisions produce neutrons that go straight on with essentially unchanged energy (half the deuteron energy) in an intense rather well collimated and monochromatic beam of neutrons. Collimated beams of these neutrons, having sucient energy, produce mesons. Moreover, 15 bev. neutrons make everything that can be made with 15 bev.- protons, and one can study the same reactions in different isotopic spin states than are available with proton bombardment. High energy acceleration of alpha particles, which are characterized by the combination of protons and neutrons, are also useful in producing neutron beams. It has additionally been advantageous to accelerate heavy particles etc.

Summary of the invention By this invention there is provided a simple and effective method for the acceleration of particles, comprising deuterons and other heavy particles in the multiple bev. range. The method and construction involved in this invention utilizes standard and well known techniques and apparatus and are highly flexible for a wide range of applications, beam energies, and heavy particles. More particularly, this invention contemplates supplying the particles from a linear accelerator source and sequentially bunching, debunching, and rebunching said particles with a narrow band, discontinuous radio frequency moving wave in a cyclic, strong focusing, flat topped magnetic confining eld. With the proper selection of injection velocity, discontinuous radio frequency and magnetic eld the required bunching, debunching, and rebunching provide for the high energy acceleration of the heavy particles in a limited RF band.

It is an object of this invention, therefore, to provide improved high energy, charged particle acceleration;

It is another object of this invention to accelerate 4heavy particles such as deuterons, alpha particles and heavy nuclei such as Lis, B10, C12 etc.;

It is also an object of this invention to provide a narrow band radio-frequency method and apparatus for the acceleration of charged particles;

It is a further object of this invention to provide a method to bunch, deb-unch, and rebunch high energy particles.

Various other objects and advantages will appear from the following description of one embodiment of this invention, and the novel features will be particularly pointed out hereinafter in connection with the appended claims.

Brief description of the drawings In the drawings where like parts are marked alike:

FIG. l is a partial cross-section of linear accelerator elements and a superimposed diagrammatic illustration of the principles involved in the acceleration of charged particles;

FIG. 2 is a diagrammatic illustration of the program and principles involved in this invention in the alternating gradient cyclic acceleration of charged particles;

`FIG. 3a is a straightened out diagrammatic illustration of the initial bunching of the particles in a cyclic acceleration ring resulting from the program illustrated in FIG. 2;

FIG. 3b is a diagrammatic illustration of the debunching of the particles of FIG. 3a;

FIG. 3c is a diagrammatic illustration of the rebunching of the particles of FIG. 3b.

Description of the preferred embodiment It is known that beams of high energy particles may be piped over long distances and accelerated by using a radio-frequency linear accelerator. A radio-frequency linear accelerator is shown and discussed in The Linear Accelerator Injector for the AGS which was printed in the IRE, 1960, International Convention Record, part 9. In FIGURE 1 of that paper which illustrates the major components of a linear standing wave accelerator, an ionized gas source having a Cockcroft-Walton pre-injector injects the particles down the axis of the accelerator and an electric eld accelerates these particles across the gaps between drift tubes having quadrupole focusing magnets therein. To this end the electric field direction reverses during each RF cycle and the particles are shielded from this electric eld when inside -the dritt tubes. When the particles emerge from the drift tubes, the electric field is again in a direction for accelerating the particles. In this arrangement the particles normally spend one RF cycle in passing from the center .of one drift tube gap -to the center of the next drift tube gap and the velocity of the particles at any point is determined by the spacing of the drift tubes.

By providing a suitable gas in the ionized gas source e.g. deuterium gas, and appropriately setting the Cock- `croit-Walton pre-injector at 375 kv., charged particles e.g. deuterons, will be supplied to the linac at about 0.02, Where c is the velocity of light, and accelerated in between the drift tubes by a suitable standing RF wave. Referring to FIG. 1, for example, which illustrates deuterons entering and passing through several drift tubes 11, the standing RF wave 13 is shielded from the deuterons when they are in the drift tubes and the deuterons take two RF cycles to pass from the center of one gap 15 between drift tubes to the center of the next gap between drift tubes, spending, therefore, most of this time within the drift tubes. The deuterons emerge from the linac with a velocity V=0.158c and an energy of mev. Exactly the same conditions are necessary for accelerating alpha particles or completely stripped heavier nuclei with Z/A=1z, to the same velocity where Z equals the proton number and A equals the nucleon number.

In accordance with this invention the particles accelerated from 0.02c to 0.1586, are injected into an alternating gradient synchrotron, such as the Brookhaven AGS, or the AGS described in U.S. Patent 2,882,396, for accelerating the particles in a ring to very high energies. In these cyclic accelerators the particles are accelerated by the travelling wave of a radio-frequency electric field which travels with the speed of the particles. The radio frequency system for this accelerator is described in further detail in The Radio Frequency Accelerating System for the Brookhaven Alternating Gradient Synchrotron by M. Plotkin, which is reprinted in the 1960 Int. Conv. Rec. Thus the particles speed is increased by increasing the wave velocity as is well known in the art. Also, strong focusing magnets bend and focus these particles along a circular orbit with a field strength corresponding to the particle velocity, and to this end the magnet strength increases correspondingly with the particle speed until the particles reach their maximum energy. The control for maintaining the proper field strength has a suitable program and servo-system having electronic probes for adjusting the field strength in accordance with the momentum of the particles and posilion of the beam.

In order to explain the function of accelerating the particles, there is an axis corresponding to the particle equilibrium orbit in the cyclic strong focusing accelerator. Disposed along this orbit Abetween the confining and focusing magnets are a plurality of suitable accelerating stations which are in phase with each other and selectively 180 out of phase with other like stations to produce the waves. These stations have ferrite loaded cavities, advantageously tuned at the lowest RF frequency with vacuum capacitors across the accelerating cavities or gaps. They are tuned over their frequency range by DC saturation such as is well known in accelerators and self-tracking amplifiers.

A particle passes one of the accelerating gaps at a point 0, designated as the stable phase angle and receives an energy increase equal to E sin 01. If the particle gains too much energy it goes faster, thereafter reaching the accelerating gap earlier at 02 and thus receiving an energy increase E sin 02 E sin 01. lf the particle gains less energy than normal, it goes slower, arrives at the gap later and there receives an energy increase E sin 03 E sin 01.

Should the RF waves be applied sequentially in stages at suitable harmonics of the particle orbital frequency and be turned ofi? in between two suitable RF wave applications, the magnets maintaining a constant field suitably in correspondance with the particle momentum while the RF waves are turned off, the particles will be bunched, debunched and rebunched. Also, by changing the harmonic number different size bunches are formed. Moreover, by suitably increasing the radio-frequency between these harmonic changes, the fields in the magnetic bending and focusing magnets increasing in correspondence with the particle momentum and having an intermediate field strength plateau when the RF is turned off, the deuterons can be accelerated to high energy in a relatively narrow or correspondingly limited band of radio frequencies.

Referring to FIG. 2, which illustrates an embodiment of these features of this invention, line 21 illustrates diagrammatically the magnetic field strength of the alternating gradient magnets for the desired particle velocities, and line 23 illustrates the discontinuous radio-frequency wave. Initially the magnetic field strength is a constant small amount advantageously 120 gauss, and the injection of the deuterons into this cyclic accelerator from the linac is in a stream of particles having substantially the same momentum. The radio-frequency voltage is programmed to start at an initially high harmonic of the particle orbital frequency and after a short time in which the particles are initially bunched, the field is increased slowly and slightly e.g, to 240 gauss. The radio-frequency voltage is also increased while locked on the initial harmonic to a point where this initial harmonic and half this harmonic lie within the predetermined limited radio-frequency band width. When this point is reached the radiofrequency acceleration is turned off and the magnetic field is kept constant, i.e. fiat topped.

This debunches the particles while the magnetic field keeps the particles circulating in storage at a constant velocity and prevents them from being lost. The radiofrequency field is then reactivated at half the frequency that was last attained, which is half the initial harmonic, for a short time sufficient to rebunch the particles. After this short time in which the particles are rebunched into larger bunches the radio-frequency field is increased rapidly while confined in this new lower harmonic of the particle orbital frequency and the magnetic field is increased correspondingly.

The time at which this change or jump in harmonic order must occur, is determined -by the condition since the initial harmonic must be below the top frequency of the band, and

VOlSc since half the initial harmonic must be above the bottom frequency of the band.

In operation, for accomplishing the acceleration of the heavy particles, such as deuterons, to 30-32 bev., the synchrotron ring is advantageously evacuated to 1()H6 millimeters of mercury or less, can contain particles up to a magnetic rigidity of 34 bv./c., and has a radius of 421.45 feet. The linear accelerator is also evacuated to a like value and has an ion source which supplies the ions e.g. deuterons in a stream.

The particles are injected into a constant magnetic field provided by lenses and bunched with the RF oscillator locked on to the beam control through the beam probes at a high harmonic of the orbital frequency at injection. These bunches correspond in size and number to this harmonic. Half the accelerating cavities are in phase and the other half are shut off since the initial acceleration takes place while B (the magnetic field) increases at a low rate. For example, the initial increase is from gauss to about 24() gauss in about 20-30 milliseconds. During this initial acceleration the RF frequency is increased until twice the injection frequency is reached. Thus about 20-30 milliseconds after injection there are achieved 24 short accelerated particle bunches 41 shown in FIG. 3a, which are stretched out in a straight line from their circular orbit for ease of explanation. These bunches still correspond in number and size to the initial harmonic.

A flat step is then put into the magnet voltage and the RF is turned off. The bea-m thus coasts and debunches into the continuous (endless) beam 43 illustrated in FIG. 3b which is a straightened View of the beam in the endless accelerator ring. The RF cavities are returned to the original frequency and the beam is rebunched in twelve longer bunches 45 as shown in FIG. 3c, the phase being locked on Ithe 12th harmonic. Then acceleration is resumed by rapidly increasing the RF and magnetic field strength. For this state (the second or 12th harmonic stage) all the RF cavities are operable and provide two times the energy gain per turn as initially (or 80 kev. instead of 40 kev.). There are advantageously six RF cavities which are in phase and six RF cavities interposed therebetween which `are out of phase. The transfer efficiency is about the same as the initial capture efficiency e.g. 75%. The phase transition is at about 14 bev. where the phase of the applied radio-frequency is shifted so that the equilibrium phase angle for the particles moves from the rising to the falling side of the RF voltage wave. This change is about 120 from 30.

To obtain the desired bunching, debunching and rebunching of deuterons within the `small band RF frequencies from 1.4 to 4.5 mc./sec. in accordance with the principles desired, the radio-frequency initially bunches 25 meV. deuterons in about 2-10 milliseconds for optimum capture at 1.4 mc./sec. and a-n injection velocity of .158c. Here the RF is advantageously the 24th harmonic of the orbital frequency, al-though other high harmonics can be used, and the RF and magnetic fields are advantageously substantially constant during bunching to provide high capture efficiencies. Then the RF is increased to 2.8 mc./sec. on this harmonic while the magnetic field is increased Ifrom 120 gauss to 240 gauss -to increase the deuteron energy to 100 mev., and the velocity to .315C in about -30 milliseconds.

The RF cuts off to debunch the particles in about milliseconds while the constant magnetic field maintains a circular particle orbit for 4a time long enough to retune to ythe 12th harmonic and to reapply the radio-frequency. For example, with a fiat topped magnetic field actual tests have shown that debunching and rebunching can easily be accomplished in 200g seconds and may advantageously be for from ZOO/.L seconds -to several milliseconds substantially without losing particles. Moreover, with a constant magnetic field at the mentioned vacuum `the particles may be stored at constant velocity for up to several minutes without losing a majority of the beam particles. On the other hand, an uninterruptedly increasing magnetic field loses a majority of the particles in the beam if the RF is turned off for 100-2001i seconds or longer.

The radio-frequency is then re-applied optimally for 2-10 milliseconds at 1.4 mc./sec. and at the 12th harmonic of the orbital frequency along with a constant field for rebunching. This is followed by increasing the RF to 4.5 mc./ sec., increasing the magnetic field from 240 gauss to about 13,000 gauss and increasing the particle velocity in this s-tage from 0.3150 to c approximately. This achieves a high energy in only about one second from injection.

Since the relation be-tween magnetic field and frequency is the same for the same e/ m, the described RF programs for deuterons, alpha particles, and heavier Z=A/2 nuclei is the same where Z=the proton number and A=the nucleon number. Thus what is stated herein with regard to the cyclic acceleration of deuterons applies also to the other above-mentioned particles. Thus, with the described strong focusing cyclic accelerator means capable of containing particles with a magnetic rigidity of 34 bv./c., the acceleration is possible with the system of this invention of 64 bev. alpha particles, 188 bev. carbon nuclei, etc.

It is understood that many different RF band of fre quencies, harmonic combinations, and numbers of stages can be used in accordance with this invention in limiting the accelerating RF to a `small or limited band of frequencies.

This invention has the advantage of accelerating particles to very high energies in a simple, efficient system employing a l-imited band of accelerating radio-frequencies. Moreover, the system of this invention employs conventional equipment and quickly and efficiently bunches, debunches and rebunches the particles for acceleration and storage. Also, this bunching and acceleration system is operable to accelerate high energy heavy particles to energies not known heretofore such as 32 bev. for deuterons, 64 bev. for alpha particles and 188 bev. for carbon nuclei.

In review of the above, the described example of this invention provides a method of accelerating charged particles heavier than protons to high energies in an alternating gradient synchrotron having magnetic means for confining and strong focusing charged particles in an endless orbit up to multiple bev. energies with a magnetic field of `from 120 to 13,000 gauss, radio-frequency accelerating first means for producing a band of accelerating frequencies from 1.4 to 4.5 mc./sec. for accelerating the particles in bunches in the synchrotron, means for energizing the first means at a harmonic of the revolution frequency of the particles in the synchrotron whereby said harmonic corresponds to the number of said bunches in said synchrotron, means for de-energizing the first means and maintaining a constant magnetic field with said magnetic means for storing the particles in the synchrotron by circulation around said endless orbit, linear acceleration means for injecting the particles into said synchrotron having a radio-frequency source of cyclic acceleration waves and drift tubes between which the particles spend at least one cycle of the waves for accelerating said particles in the linear acceleration means to an energy of at least 25 mev., the number of cycles of the waves that the particles spend in the drift tubes depending on the injection energy and velocity of the particles in-to the linear acceleration means, and charge particle source means for the linear acceleration means having a pre-injector capable of injecting the particles into the linear acceleration means at energies up to at least 375 kv., comprising steps of:

(1) Injecting charged particles heavier than protons from the preinjector into the linear acceleration means at an energy of 375 kv. and a velocity of 0.02 the speed of light whereby the heavy particles take two cycles of the waves to pass between the drift tubes and emerge from the linear acceleration means with a velocity of 0.158 the speed of light and an energy of 25 meV.;

(2) Injecting the heavy particles into the synchrotron and circulating them therein in a magnetic field of 120 gauss produced by 4the magnetic means;

(3) Increasing the frequency of the first means from 1.4 to 2.8 mc./sec. at the 24th harmonic of the revolution frequency of the heavy particles in the synchrotron while increasing the field produced by the magnetic means from 120 to 240 gauss to accelerate the heavy particles in the synchrotron lto an energy of 100 mev. and a velocity of .315 the speed of light;

(4) Cutting off the first means while circulating the heavy particles in lthe synchrotron in a magnetic field of 240 gauss with the magnetic means; and

(5) Increasing the frequency of the first means from 1.4 -to 4.5 mc./sec. at the 12th harmonic of the revolution frequency of the heavy particles in the synchrotron while increasing the magnetic field produced by the magnetic means from 240 to 13,000 gauss to accelerate the heavy particles to multiple bev. energies and velocities of .99 the speed of light.

I claim:

1. The method of increasing the revolution frequency and energy of electrically charged particles that are held on a revolution orbit in a synchrotron from low energies to multiple bev. energies wherein these particles are exposed at at least one location of said orbit to a high frequency electric field the frequency of which increases in correspondence with the increasing of the revolution frequency of the particles, characterized in that the particles are injected into said synchrotron from a linear accelerator having `seri-ally arranged drift tubes through which said particles pass and an alternating cycle high frequency electrical energy source with which said particles are accelerated in between said drift tubes, said particles spending two cycles of said alternating cycle high frequency electrical energy source .in each of said drift tubes, and said particles are exposed in said synchrotron to a high frequency electric field the frequency of which is successively made equal to different harmonics of the revolution frequency with decreasing ordinal numbers of the harmonics.

2. The method according to claim 1, characterized in that at the change from the higher to the lower harmonic in the synchrotron the high frequency electric field is cut off during a time interval in which the particles are held on the revolution orbit by a constant magnetic field.

3. The method of claim 1, characterized in that the high frequency electric field is initially equal to the 24th harmonic of the revolution frequency of the particles in the synchrotron and then is equal to the 12th harmonic of the revolution frequency of the particles.

4. The method of claim 1, characterized in that the frequency of the high frequency electric field is initially increased in the synchrotron from 1.4 nic/sec. to 2.8 mc./sec. at the 24th harmonic of the revolution frequency of the particles, said particles being deuterons and their energy thereby being increased from 25 mev. to 100 mev. after injection into said synchrotron, is cut off during a time interval in which the particles are held on the revolution orbit by a constant magnetic field, and then is increased from 1.4 mc./sec. to 4.5 mc./sec. at the 12th harmonic of the revolution frequency of the particles thereby increasing the energy of the deuterons from 100 mev. to 30 bev., each of the respective harmonics corresponding to the frequency of the passage of the particles past said one location on a predetermined constant revolution orbit forming a circle whereby said particles pass said one location with the proper difference between the angular velocity of the particles and the angular frequency of the high frequency electric field.

5. The method of claim 1, characterized in that the electric field initially has a frequency of from 1.4 to 2.8 mc./sec. corresponding to the 24th harmonic of the revolution frequency of the particles in the synchrotron to produce twenty-four discrete bunches of particles moving around the circumference of said revolution orbit with the particles passing said one location at a stable phase angle point 0 to receive an energy increase equal to E sin 01, thereby to increase the velocity of said particles from .158 to .315 the speed of light, and thereafter said electric field has a frequency of from 1.4 to 4.5 mc./ sec. corresponding to the 12th harmonic of the revolution frequency of the particles to produce twelve discrete bunches of particles moving around the circumference of said revolution orbit with the particles passing said one location Iat a stable phase angle point 0 -to receive an energy increase equal to E sin 01 thereby to increase the velocity of said particles from .315 to close to the speed of light whereby the velocity of particles heavier than protons can be increased from .158 the speed of light approximately to .99 the speed of light with a narrow band of electric field frequencies between 1.4 and 4.5 mc./sec.

6. The method of claim 1 in which said particles are selected from the group consisting of deuterons, alpha particles and completely stripped heavier nuclei with Z/A=1/z, where Z equals the proton number and A equals the nucleon number.

No references cited.

JAMES W. LAWRENCE, Primary Examiner.

R. JUDD, Assistant Examiner. 

